Final answer:
Sorting operators are not an extended operator of relational algebra; the extended operators include duplicate-elimination operator, extended projection, and natural join operator.
Step-by-step explanation:
Among the options given, sorting operators are not considered an extended operator of relational algebra. Relational algebra primarily consists of a set of operations that are used on relations (or tables) to retrieve or derive relations according to a query. The classical set of operators include selection, projection, union, set difference, Cartesian product, and rename. Extended operators enhance the capabilities of relational algebra, such as the duplicate-elimination operator (d), extended projection (which includes arithmetic operations in addition to column projection), and the natural join operator, which is a specialized form of the join operation to combine relations in a specific manner.